Firstly, I would like everyone to check out this article by Uilleam Blacker published in History Today, which does an excellent job of explaining the ideas and effects of Malorossicism in detail. One telling passage in particular:
In July 2021 Putin published a lengthy essay, ‘On the Historical Unity of the Russians and Ukrainians’, in which he aired multiple grievances against Ukrainians, while simultaneously denying their existence as a separate nation. He reached the contradictory conclusion that ‘genuine sovereignty for Ukraine is only possible in partnership with Russia’. Just as Catherine did in the 18th century, Putin proposes to bring order to what he views as an unruly borderland.
For our third installment, we move on to Ivan III, the Grand Prince of Moscow from 1462-1505. At the time, much of Ukraine and Belarus were not controlled by Moscow or any of the other remaining Russian principalities or, in the case of Novgorod, republics, but rather by the Grand Duchy of Lithuania, whose initial growth in response to attempted expansion by the Teutonic Knights had expanded into a quest to “Gather the Lands of Rus’ as Lithuania itself increasingly Ruthenized, going so far as to adopt Old Ruthenian as the language of government until its formal dissolution in 1791. Eminent Ukrainian historian (and President) Mihailo Hrushevsky argued that the Grand Duchy had preserved the traditions and forms of Kyivan Rus’.
Ivan III had different ideas. Writing to Grand Duke Alexander Jagiellon, he established a new tradition of presumptive supremacy. Here is a summary taken from part of Norman Davies’ Europe: A History
In January 1493 Moscow's relations with Lithuania were about to take a new turn. Six months earlier Casimir Jagiellonczyk, King of Poland and Grand Duke of Lithuania, had died, dividing his realm between his second and third sons. The Polish kingdom passed to Jan Olbracht, Lithuania to the unmarried Alexander. (The eldest son was already King of Bohemia and Hungary.) Ivan III had seen the possibilities. On the one hand, he was preparing an embassy which would travel to Wilno, and would launch negotiations leading to a political marriage between Grand Duke Alexander and Ivan's daughter Elena. At the same time, he was setting conditions which would undermine the previous modus vivendi of the two states. For the first time in Moscow's history, he armed his ambassador with instructions demanding recognition of the hitherto unknown title of gosudar' vseya Rusi — 'lord of all-Rus'. It was a classic diplomatic double hold — one part apparently friendly, the other potentially hostile. Ivan was deliberately pulling Lithuania into an engagement that called into question the future of all the eastern Slavs.
The title 'Lord of All-Rus'' did not possess much basis either in history or in current reality. It came into the same category as that whereby the kings of England laid claim to France. In the 1490s, two-and-a-half centuries after all traces of a united Kievan Rus' had been destroyed, it had the same degree of credibility that the king of France might have enjoyed if, in his struggle with the German Empire, he had proclaimed himself 'Lord of all the Franks'. By that time, it conflicted with the separate identity that the 'Ruthenes' of Lithuania had assumed from the 'Russians' of Moscow. Indeed, it all seemed sufficiently unreal for the Lithuanians to accept it as a small price to pay for Ivan's good humour. They were not to know it, but they were conceding the ideological cornerstone of territorial ambitions that would be pursued for 500 years.
Essentially, Ivan III claimed sovereignty over all the territories that had been Kyivan Rus’ supported not by actual control, but rather based on race and history. It should be noted that the Lithuanians were not particularly interested in Ukrainian sovereignty (in fact, the Union of Lublin would cede Ukraine to the Poles, starting a tension and rivalry that served as counterpoint to the Ukrainians’ history with Russia) and that “All the Russias” infer that there was such a thing as a “Little Russia” separate from his own “Great Russia”. That being said, Ivan’s claims mark a significant sea change in Russian policy regarding Ukrainian sovereignty, and his wars against Lithuania and Novgorod would begin the process of achieving his claims. It would largely be up to his successors to make these ambitions a reality.